1. Field of the Invention
The present invention relates to the improvement of an electric motor having a moving part made up of a permanent magnet; the electric motor is used for driving or positioning various kinds of industrial machinery, that is, the construction machinery, processing machinery, OA instruments, machine tools, and precision instruments and so forth.
2. Description of the Prior Art
A conventional electric motor, for instance, is equipped with a rotor which is mounted on a frame by means of an axle so as to revolve freely and also is made of a permanent magnet that is magnetized in a diametrical direction, and with a stator which forms, in a concentric way with the rotor, the outer circumference of the rotor, and which is provided with field poles standing out on its surface and having equal spaces in the direction of the circumference of the yoke, and each of the field poles has a coil wound around it. The yoke and the field pole are each made of soft magnetic material or, alternatively the yoke and the field pole are combined into a monolithic part, also being made of soft magnetic material.
This kind of the electric motor is given a one-step turn in the regular direction by attracting the magnetic pole of the rotor to the magnetic field of the field pole excited by the excitation of the coil, and thereafter the excited coil is made free from excitation, causing the next coil in the revolving direction to be excited in turn so as to attract the magnetic pole of the rotor to the magnetic field of the field pole, giving another one-step turn. In this way the production of the shifting magnetic field around the field pole by exciting each of the coils in regular turn enables the rotor to revolve continuously.
Furthermore when the rotor is required to stop in a certain position which is decided previously, with respect to the conventional electric motor, a holding current flows in the coil in order to keep a rotor in a required position by the holding torque produced when attracting the magnetic pole of the rotor to the magnetic field of the magnetic pole.
The structure of the conventional electric motor, however, needs an exciting current which excites the field pole nearly up to the saturation range so as to produce the shifting magnetic field.
Also the structure of the conventional electric motor needs to keep the holding current flowing, when the rotor is required to stop in a position which is decided previously. Therefore, when the conventional electric motor sends power or moves an object to a regular position correctly, it needs an operating structure such as a ring of gears and a link mechanism.